I've only completed the FF series so far. I can recommend the following for a good challenge:
CAR:
Honda Integra Type R (DC2) '95, bone stock except for oil change. 201 BHP.
TYRES:
Tsukuba = Comfort Soft
Eiger = Comfort Soft (Though I think impossible if the field of cars has a Civic start the race in lead - in this case exit completely then re-enter to get a different line-up of opponents or upgrade to Sport Hard)
Trial Mountain = Comfort Medium
I'm pretty sure these would all be do-able without the oil change but havn't had time to try that. Oh, and all aids off except ABS on 1 by the way.
CAR:
Honda Integra Type R (DC2) '95, bone stock except for oil change. 201 BHP.
TYRES:
Tsukuba = Comfort Soft
Eiger = Comfort Soft (Though I think impossible if the field of cars has a Civic start the race in lead - in this case exit completely then re-enter to get a different line-up of opponents or upgrade to Sport Hard)
Trial Mountain = Comfort Medium
I'm pretty sure these would all be do-able without the oil change but havn't had time to try that. Oh, and all aids off except ABS on 1 by the way.